全文获取类型
收费全文 | 75647篇 |
免费 | 13361篇 |
国内免费 | 20665篇 |
专业分类
化学 | 54204篇 |
晶体学 | 2594篇 |
力学 | 5287篇 |
综合类 | 2231篇 |
数学 | 11337篇 |
物理学 | 34020篇 |
出版年
2024年 | 207篇 |
2023年 | 933篇 |
2022年 | 2576篇 |
2021年 | 2596篇 |
2020年 | 2555篇 |
2019年 | 2526篇 |
2018年 | 2274篇 |
2017年 | 3107篇 |
2016年 | 2721篇 |
2015年 | 3619篇 |
2014年 | 4334篇 |
2013年 | 5846篇 |
2012年 | 6064篇 |
2011年 | 6491篇 |
2010年 | 5948篇 |
2009年 | 6080篇 |
2008年 | 6839篇 |
2007年 | 6069篇 |
2006年 | 5854篇 |
2005年 | 5145篇 |
2004年 | 3957篇 |
2003年 | 2952篇 |
2002年 | 2890篇 |
2001年 | 2812篇 |
2000年 | 2802篇 |
1999年 | 1810篇 |
1998年 | 1144篇 |
1997年 | 968篇 |
1996年 | 958篇 |
1995年 | 816篇 |
1994年 | 879篇 |
1993年 | 817篇 |
1992年 | 727篇 |
1991年 | 559篇 |
1990年 | 511篇 |
1989年 | 508篇 |
1988年 | 423篇 |
1987年 | 335篇 |
1986年 | 301篇 |
1985年 | 279篇 |
1984年 | 221篇 |
1983年 | 197篇 |
1982年 | 184篇 |
1981年 | 150篇 |
1980年 | 122篇 |
1979年 | 120篇 |
1978年 | 64篇 |
1976年 | 52篇 |
1974年 | 54篇 |
1973年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
A convenient and large-scale preparation of retinoic acid 1 from β-ionone in five steps with 38% overall yield is described.The key steps are the epoxidization of 2 with a new methylated agent and the condensation 4 with tetraethyl methylenediphophonate in one-pot procedure to prepare 6. 相似文献
93.
The structure of the title adduct comprises a phenanthroline derivative 2-phenyl-imidazo[4,5-f]1,10-phenanthroline and a methanol.The composition of the crystalline adduct was characterized as C19H12N4.CH3OH.It belongs to orthorhombic system,space group Pna21 with a=1.3693(4)nm,b=2.2988(7)nm,c=0.51338(15)nm,V=1.6160(8)nm^3.Z=4,and final R1=0.0423.wR2=0.1012 .Crystal structure shows that all the 19 carbon atoms and 4 nitrogen atoms are coplanar.The bond length data indicated that a very extensive conjugation system was formed.This conjugation makes the compound being a potentially excellent energy transformer used for luminescent materials. 相似文献
94.
We establish some liminf theorems on the increments of a (N,d)-Gaussian process with the usual Euclidean norm, via estimating upper bounds of large deviation probabilities on the suprema
of the (N,d)-Gaussian process.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
95.
The properties of pulsed laser vapor doping on p-Si(1 0 0) with a KrF (248 nm) excimer pulsed laser (248 nm) and BCl3 gas are reported in this paper. The doped samples are characterized by the resistance measured using a four-probe method, since the sheet resistance changes with the carrier concentration of the sample. The doping effects with the variation of laser energy density, pulse number, and the pressure of BCl3 were investigated in terms of the sheet resistance. In this way, the optimized parameters were obtained and used for the positive heavy doping on p-Si(1 0 0) and p-Si(1 1 1). Then, using a square mesh under the above conditions, an image doping was completed. Finally, the metal–semiconductor Ohmic contacts were realized by plating Ag and Cu films on the doped surface. 相似文献
96.
The growth mechanism of the peritectic η phase involving the peritectic reaction and peritectic transformation in Cu-70%Sn alloy was investigated under directional
solidification. The results show that a major growth mechanism in thickening of the peritectic η-layer is not the peritectic reaction but the peritectic transformation. The transformation temperature and isothermal time
play crucial roles in determining the volume fraction and the thickness of the peritectic η phase. With the increase of the temperature and isothermal time, the volume fraction of the peritectic η phase increases. The regressed data show that the relationship between the thickness of η phase (Δx) and the transformation temperature (T) meets the following equation In Δx=6.5−1673 1 / T. Additionally, there exists a relationship between the thickness of the η phase (Δx) and the isothermal time (t) at the 9 mm solidification distance below the peritectic reaction interface, Δx=0.72t
1/2, which is consistent with the theoretical model.
Supported by the National Science Foundation of China (Grant No. 50395102) 相似文献
97.
We study maps from a 2‐surface into the standard 2‐sphere coupled with Born‐Infeld geometric electromagnetism through an Abelian gauge field. Such a formalism extends the classical harmonic map model, known as the σ‐model, governing the spin vector orientation in a ferromagnet allows us to obtain the coexistence of vortices and antivortices characterized by opposite, self‐excited, magnetic flux lines. We show that the Born‐Infeld free parameter may be used to achieve arbitrarily high local concentration of magnetic flux lines that the total minimum energy is an additive function of these quantized flux lines realized as the numbers of vortices antivortices. In the case where the underlying surface, or the domain, is compact, we obtain a necessary sufficient condition for the existence of a unique solution representing a prescribed distribution of vortices antivortices. In the case where the domain is the full plane, we prove the existence of a unique solution representing an arbitrary distribution of vortices and antivortices. Furthermore, we also consider the Einstein gravitation induced by these vortices, known as cosmic strings, establish the existence of a solution representing a prescribed distribution of cosmic strings cosmic antistrings under a necessary sufficient condition that makes the underlying surface a complete surface with respect to the induced gravitational metric. © 2003 Wiley Periodicals, Inc. 相似文献
98.
有限群表示的一个结论 总被引:1,自引:0,他引:1
张玉成 《数学的实践与认识》2004,34(3):116-119
利用陪集、重陪集等概念和性质 ,证明了一个利用有限群 G的子集表示 G的结论 . 相似文献
99.
PURE STATE APPROACH TO C(x)_αZ_nLIBINGREN;LINQING(InstituteofMathematics,AcademiaSinica,Beijing100080,China.Projectsupportedbyth?.. 相似文献
100.
利用无穷维李代数方法得到了相互作用sl玻色子体系在U( 2l+ 1 ) O( 2l+ 2 )过渡区的能谱和波函数的严格解 .给出了该系统Bethe假定方程的数值解法 . 相似文献